catena-Poly[[bis(diaqualithium)]-μ4-3,3′,5,5′-tetranitro-4,4′-bipyrazole-1,1′-diido]: a new moisture-insensitive alkali-metal energetic salt with a well-defined network structure

In the title salt, the 3,3′,5,5′-tetranitro-4,4′-bipyrazole-1,1′-diide dianion [{TNBPz}2−] is situated across the twofold axis. The distorted coordination octahedra around Li+ involve four short bonds with two pyrazolate N atoms and two aqua ligands and two longer contacts with nitro-O atoms. When combined with μ4-{TNBPz}2−, this generates a mono-periodic polymeric structure incorporating discrete centrosymmeric [(H2O)2Li–(dinitropyrazolato)2–Li(H2O)2] units. The three-dimensional stack of mutually orthogonal coordination chains is reminiscent of a Lincoln log pattern.

In the structure of the title salt, [Li 2 (C 6 N 8 O 8 )(H 2 O) 4 ] n , the 3,3 0 ,5,5 0 -tetranitro-4,4 0 -bipyrazole-1,1 0 -diide dianion [{TNBPz} 2À ] is situated across the twofold axis. The distorted coordination octahedra around Li + involve four short bonds with two pyrazolate N atoms and two aqua ligands  (15) and 3.0887 (15) Å ]. The Hirshfeld surface and twodimensional fingerprint plots also support the significance of non-covalent bonding. Coordinative saturation and a favorable geometry at the Li + ions, dense packing of the polymeric subconnectivities and particularly extensive interanion interactions may be involved in the stabilization of the structure. The title salt is a rare example of an energetic Li nitroazolate, which nicely crystallizes from aqueous solution and is neither hygroscopic nor efflorescent. The TG/DTA data reveal total dehydration in the range of 330-430 K and stability of the anhydrous material up to 633-653 K.

Chemical context
Red-light-emitting technical or military pyrotechnics traditionally concern the utilization of Sr salts (Sabatini, 2018). However, there is a growing interest for alternative red-flame colorants since strontium is potentially harmful to human health, specifically replacing calcium in bone and affecting skeletal development (Glü ck et al., 2017). Recent works by Klapö tke suggest significant potential for lithium-based systems, in particular those incorporating energetic nitropyrazole species (Dufter-Mü nster et al., 2022;Dufter et al., 2020). The accumulation of nitro groups enhances acidity (pK a = 3.14 for 3,5-dinitropyrazole vs 14.63 for the parent pyrazole; Janssen et al., 1973) for producing hydrolytically stable salts, while the incorporation of energy-rich nitropyrazolates contributes to oxygen balance of the formulations. In addition, the high nitrogen content and N-N linkage within the pyrazole ring inherently facilitate the release of nitrogen gas when burned. This meets the needs for cooling the flame for improving the color purity (Glü ck et al., 2017).
However, most of the examined salts are still not suited for applications in spite of such valuable pre-requisites. The nitropyrazolates crystallize with difficulty (Drukenmü ller et al., 2014) and their Li salts are commonly hygroscopic and deliquescent (Dufter-Mü nster et al., 2022). In the present work, we address this problem with a crystal-engineering approach. The recently introduced bifunctional tecton 3,3 0 ,5,5 0 -tetranitro-4,4 0 -bipyrazole [H 2 (TNBPz)] readily affords a range of salts with alkali metal ions (Domasevitch & Ponomarova, 2021) and nitrogen bases (Gospodinov et al., 2020) and supports either coordination or hydrogen-bonded arrays in a very predictable fashion. One can anticipate that the doubled nitropyrazolate functionality could grant the connection of the Li + ions and generation of a relatively robust polymer, whereas the extended molecular framework of {TNBPz} 2À is particularly beneficial for the dense anionanion interactions because of a larger contribution of dispersion forces. An appropriate set of binding sites for such interactions may be found with four NO 2 functions, which commonly act as self-complementary donor and acceptor groups for non-covalent lone pair-hole bonds (Bauzá et al., 2017). With this in mind, we prepared the new energetic salt catena-poly [[bis(diaqualithium)]-4 -3,3 0 ,5,5 0 -tetranitro-4,4 0 -bipyrazole-1,1 0 -diido] and report its structure here.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1, with the asymmetric unit comprising one metal ion, two aqua ligands and half a molecule of the organic dianion {TNBPz} 2À , which is situated across the twofold axis passing through the center of the C-C bond between two pyrazole rings.
The coordination around the Li + ion may be regarded as distorted octahedral, with four relatively short bonds with two pyrazole-N atoms [2.086 (2) and 2.090 (2) (Olsher et al., 1991), the spread of the bond lengths is usually narrower. For example, the citrate salt exhibits six Li-O bonds in the range of 1.998 (2)-2.222 (3) Å (Rossi et al., 1983). The exceedingly long bonds with nitro groups may be described rather as very weak ion-dipole contacts, while the remaining four shorter bonds almost perfectly match the sum of the corresponding ionic radii for 4-coordinate Li + ions [which are Li-O = 1.94 Å and Li-N = 2.05 Å ; Shannon, 1976]. Nevertheless, the weak Li-O(nitro) interactions are presumably important for a more effective shielding of the cations against hydration when exposed to moist air. Unlike many related systems, crystals of the title compound are not hygroscopic. A second issue is the saturation of the Li + environment with an appropriate number of aqua ligands. This is contrary to the structures of far more moisture-sensitive 3,4-, 3,5-dinitropyrazolates and 4-amino-3,5-dinitropyrazolate, where the diaqualithium moieties were recognized as local fragments of 1:1 aqualithium chains -Li-[(-H 2 O)Li] n - (Dufter-Mü nster et al., 2022).

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Dotted lines indicate distal Li-O(nitro) interactions. Two out of four H atoms of the aqua ligands are equally disordered over two positions (H2A, H2B and H4A, H4B).

Supramolecular features
The title compound adopts a mono-periodic polymeric structure with the {TNBPz} 2À anions acting as tetradentate bridging ligands. Two dinitropyrazolate groups of the anions and two diaqualithium fragments compose the cyclic pattern ( Fig. 2), which is reminiscent of the dimers in lithium 3,5dinitro-4-amino-and 3,4-dinitropyrazolates (Dufter-Mü nster et al., 2022). Unlike these monofunctional prototypes, with the C2-C2 ii bond linking the two pz halves of the bipyrazole core, these dimers are connected into linear chains, with a distance of 9.06 Å between the centroids of the Li 2 (pz) 2 cycles (pz is pyrazole). Adjacent chains are linked by a set of conventional hydrogen bonds O-HÁ Á ÁO, which involve either aqua or nitro-O acceptors. The geometric parameters for five types of such interactions are very comparable (  (Fig. 3).
cell. In spite of the twisted conformation of the bipyrazole, such stacking is geometrically favorable, with every pair of molecules within the stack mutually fitting like puzzles. The resulting interactions are particularly extensive, with four pairs of symmetry-related short contacts N3Á Á ÁO4 iii = 3.0349 (15) Å and N2Á Á ÁO2 vi = 3.0887 (15) Å [symmetry codes: (iii) Àx + 1 2 , y, z + 1 2 ; (vi) x, Ày + 1 2 , z À 1 2 ] established by every {TNBPz} 2À anion (Fig. 4). For the mutually bonded nitro groups, i.e. N3O1O2 and (N4O3O4) iii , the latter is lone-pair donor and the former one is -hole acceptor, which combine to create a very characteristic stack (Veluthaparambath et al., 2023). The planes of the two groups subtend a dihedral angle of 34.16 (15) , but the N3Á Á ÁO4 iii axis is nearly orthogonal to the acceptor plane, as indicated by a slippage angle of 8.6 (2) . The second type of interaction of is a lone pair--hole bond with the dinitropyrazolate ring system. Similar interactions are well known for electron-deficient N-heterocycles and they are most pronounced for 1,2,4,5-tetrazines (Gural'skiy et al., 2009). In this case, the interplanar [49.96 (10) ] and slippage angles [13.98 (15) , with respect to the centroid of the pyrazole ring] are slightly larger. This non-covalent bonding is clearly traced in every structure adopted by H 2 (TNBPz) [with very short mutual nitro contacts down to NÁ Á ÁO = 2.9115 (15) Å ; Domasevitch et al., 2019] and its anions (Domasevitch & Ponomarova, 2021) and in fact it may be regarded as a prominent feature for the crystal chemistry of such systems. These close interactions of shape-complementary twisted molecules contribute to the relatively high packing index of 75.8%, which is at the top of the 65-75% range expected for organic solids (Dunitz, 1995).

Figure 5
Hirshfeld surface of the individual {TNBPz} 2À anion, mapped over d norm (the C-H distances are normalized) in the color range À0.73 (red) to 1.14 a.u. (blue), with the red regions indicating the sites of intermolecular interactions.

Figure 3
The hydrogen bonding between adjacent coordination chains, which is shown by the dotted red lines. Two successive layers are marked in blue and gray and the Li + ions are presented as coodination tetrahedra, while considering only the four shortest coordination bonds for clarity. The purple lines identify the directions of the coordination chains, which coincide with the crystal directions (110) and (110). [Symmetry codes: (vi) x, Ày + 1 2 , z À 1 2 ; (vii) Àx + 3 2 , Ày + 1 2 , z; (ix) x + 1, y, z; (x) x + 1 2 , y + 1 2 , Àz.] but they are still detectable on the surface as a set of very diffuse spots (À0.04 a.u.). The two-dimensional fingerprint plots (Fig. 6) are even more informative. They suggest the significance of coordination and hydrogen-bonding interactions, which are reflected as two sharp spikes pointing to the lower left with the shortest contacts NÁ Á ÁLi = 2.1 Å and OÁ Á ÁH = 2.0 Å . One can note a similar indication of NÁ Á ÁLi (9.6%) and OÁ Á ÁLi (4.5%) contacts, but the fraction of the latter is significantly less and the corresponding short spike is diffuse. This agrees with the weakness of the coordination bonds adopted by the nitro-O atoms. OÁ Á ÁH interactions account for 40.1% of the entire number of contacts. This is complemented by a 10.2% contribution of NÁ Á ÁH contacts, but there are no signs of any O-HÁ Á ÁN bonding. The plot represents a rather diffuse collection of points with the shortest NÁ Á ÁH = 2.8 Å . The large fraction of OÁ Á ÁN/NÁ Á ÁO and OÁ Á ÁC/CÁ Á ÁO contacts (in total over 20%) is a primary indicator for extensive anion-anion interactions. The nature of these contacts is similar and they appear in the plots as nearly symmetrical (about the diagonal where d i = d e ) pairs of features. Therefore, either donor or acceptor sites of the bonds are found within the individual anions supporting the shortest contacts OÁ Á ÁN = 3.0 Å and OÁ Á ÁC = 3.1 Å . It may be postulated that the accessibility of aqua hydrogen-bond donors does not disrupt the main anionanion interactions, but rather governs elimination of less favorable nitro OÁ Á ÁO contacts. The total contributions of the C(N)Á Á ÁC(N,O) contacts in the title structure and in the very similar unsolvated Rb{H(TNBPz)} (Domasevitch & Ponomarova, 2021) are nearly identical (33.4% and 31.9%, respectively), while the impact of hydrogen bonding is best illustrated by the pronounced contraction of the OÁ Á ÁO fraction (37.4% to 11.2% in the present case). Moreover, the asymmetry of the OÁ Á ÁO plot is contrary to the patterns for the OÁ Á ÁN/NÁ Á ÁO and OÁ Á ÁC/CÁ Á ÁO contacts. This witnesses the prevalence of the nitro/aqua contacts, instead of direct nitro OÁ Á ÁO interactions.
For the preparation of the title compound, 0.294 g (7.0 mmol) of LiOHÁH 2 O was dissolved in 10 ml of water at 333-343 K and then 1.162 g (3.5 mmol) of solid H 2 (TNBPz)ÁH 2 O was added with stirring. The mixture was stirred for 30 min and the resulting clear deep-yellow solution was cooled to r.t. Slow evaporation to a minimum volume over 8-10 d led to crystallization of the product as well-developed large lemon-yellow prisms. The crystals were removed and dried on a filter paper in air. The yield was 1.25 g (90%). The material shows neither signs of hygroscopy nor efflorescence when exposed to ambient air for months.
Analysis ( The FT-IR spectrum reveals a distinctive pattern, which is characteristic for hydrated nitropyrazolates. The peaks, which are associated with the aqua ligands, appear at 3460 and 3580 cm À1 (O-H stretching), 1641, 1658 cm À1 (bend) and 564 cm À1 (libration). The peaks for symmetric and asymmetric NO 2 stretching (1351, 1381 and 1484, 1540 cm À1 , respectively) are very similar to the spectra of comparable 3,5-dinitropyrazole (Ravi, 2015). These double peaks originate in coupling of the NO 2 vibrations with the ring motions. The intense and sharp band at 853 cm À1 is (C-NO 2 ), and its shift, with respect to the band for H 2 (TNBPz)ÁH 2 O (839 cm À1 ; Domasevitch et al., 2019), suggests a certain increase of conjugation of the nitro groups with the carrier aromatic ring upon deprotonation. For Rb{H(TNBPz)}, both these frequencies were present (839 and 852 cm À1 ; Domasevitch & Ponomarova, 2021).
Preliminary assays for safety of the title compound and its suitability for pyrotechnic formulations were performed by thermal analysis (OZM Research DTA 552-Ex). There are  two partially separated stages for nearly identical weight losses in the temperature range of 330-430 K (Fig. 7), which correspond to total dehydration of the salt (in total, À18.49 mass %; À4H 2 O: calculated À18.09%). The anhydrous material is stable up to 633 K, with the very minor exothermic event at 597 K possibly indicating a phase transition. Exothermic decomposition proceeds above 653 K, with instantaneous loss of any remaining weight and a sharp exothermic effect at ca 700 K suggesting an explosion. For comparison, typical onset temperatures for decomposition of energetic Li nitropyrazolates are 400-500 K, and only 3,5-dinitropyrazolate is stable up to 600 K (Dufter-Mü nster et al., 2022).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )